Proceedings of the 4th African Rift Geothermal Conference Nairobi, Kenya, 21-23 November 2012

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Anthology of Geothermal Power Plants Efficiency: Energy Recovery and Water Condensate

Recovery

Hugo Fernando Navas

eems international®, ,8 calle 17-15 Zona 15 Colonia El Maestro 2, Guatemala City, GUATEMALA

hfnavas_lion@yahoo.com

Keywords: Energy factors and conversions MWe, MWh,

t/h to kg/s, enthalpy kJ/kg to KWh/t, energy balance, energy

recovery, steam condensate mass loss, 4 areas of a modern

geothermal power plant.

ABSTRACT

This work is titled “Anthology” for being a compendium of

selected concepts related to a geothermal energy power

plant, and ultimately its operating efficiency.

First, before discussion of the concepts, methods, and

technologies of “energy recovery” there must be a clear

consciousness of the “total energy balance” involved in the

energy conversion processes of a geothermal power plant.

For example, as shown in Table 1 below the total energy is

distributed through several surface processes. To generate

100 MWe, the total energy carried by the geothermal fluid

produced is in the order of 850 MWh of thermal energy.

Table 1: Example of a traditional design total energy

balance

Process Description MWh %

Geothermal fluid produced from wells 850 100

Separated steam, 60 % mass 700 83

Separated brine, 40 % mass 150 17

Transmission + transportation loses 125 15

Separation + venting + other losses 50 6

Vacuum system ejectors 35 4

Plant inlet to turbine – generator 500 60

Plant outlet to condenser 425 50

Cooling system, 70% of steam mass is

lost to ambient with conventional cooling

400 45

Delivery to National grid 100 MWe 100 12

Therefore to produce 1,000 MWe the surface energy

extracted from the geothermal field is 8.5 GWh, every hour.

Keep this ratio in mind. - usually a hidden/obscure fact.

Secondly, for purposes of clarity, unification, and better

communication – for the geographical geothermal field and

power plant - there should also be in place a modern

geothermal power plant, design / planning / organizational

concept, in 4 areas. There is need to have a common field

and plant concept. This is just as important as having a

units system. A geothermal power plant can be defined as

4 areas:

A1. Reservoir + wells for production or reinjection +

steam gathering + separation + transportation + venting.

A2. Energy conversion plant: turbine + generator +

condenser + vacuum + cooling system + pumps + fans.

A3. Electric yard / delivery to grid: Substation +

transformer + tower + meter.

A4. Control room + auxiliaries. Operators control +

maximize production.

Thirdly, there is need to adopt the P h chart to modern

geothermal terms. When Richard Mollier, in late 1800s

deduced his enthalpy concept, the units and charts; he was

probably thinking of fueled steam boilers conditions of

water, not geothermal power plants. To adapt this to

modern geothermal terms the unit of h: [kJ/kg] can be

expressed as [KWh/t] - an obvious convenience in

geothermal terms.

Fourthly,, the quantity of wasted condensate water in the

traditional cooling system, is 70% of the separated steam

mass (5.6 t/MWe) which is unacceptable owing to the fact

that EAR countries are dry zones and need this water.

5.6 x 8,400 h/yr x 30 yr = 1.4 million m3 / MWe

700 million m3 / 500 MWe. (= Lake Naivasha)

1,400 million m3 / 1,000 MWe.

This is a lot of water lost with no benefit. Isn’t it? It

would rather be used for drilling + reinjection for reservoir

sustainability + other direct uses.

Now, with this background we can think about and tackle

the issues of where and how to achieve efficiency. How to

recover some wasted energy, how to recover wasted water

condensate, economically, and sustainably while making

money at the same time. WIN WIN WIN.

There are developed methods and technologies to recover

some lost energy (4 – 10%), and others to recover the lost

steam condensate water (up to all the lost condensate).

1. INTRODUCTION

There are few works, related to overall geothermal power

plant efficiency and how to enhance it. But the actual

situation is that, and most of the thermal energy produced

on the surface from the geothermal fluids goes to waste.

However if we study and understand the processes, it can

be improved. Efficiency is Profitable.

Energy recovery requires some planning, execution and

minor modifications. It is an economical way of producing

more MWe and making money.

Steam condensate water recovery requires some planning

and execution. Specific A2 system components may be

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upgraded, modified, or replaced; value of water defined and

a suitable technology selected.

2. TOTAL ENERGY BALANCE

As already stated, it is important to keep the overall energy

picture in mind, since the subject of a geothermal energy

power plant is energy - before during and after all its

conversions. The practical ratio for condensing power

plants is 850 MWh thermal to 100 MWe.

Step one: Prepare a study to know the overall efficiency,

and have an energy balance of the actual conditions of a

specific geothermal power plant.

MWh total + MWe + % efficiency

Then work from there up to higher economic efficiency.

If a condensing power plant has 12% operating efficiency,

it is possible to increase it to 14 %. Energy Recovery is

economical, with minor modifications in the plant, no

drilling, no new wells. It basically involves using the

energy on the surface better than initially designed.

2.1 Practical Limit of Energy Recovery

It is reasonable to expect an energy recovery plan to range

from 2 – 10 % of the total energy, depending on the present

efficiency and identified sources of wasted energy.

Some of the energy used / not used on the surface is an

inevitably high % of the total, but it is good to know exactly

where, when, and why.

2.1.1 Transmission Losses: It is inevitable to have some cost or loss, due to

transportation of energy from one point to another. In a

geothermal power plant it is common to have several km of

steam pipe lines going from wells to the power plants, after

steam separation. 35km of steam pipe lines in each

geothermal field of about 150 MWe can be normal.

Careful design and actual evaluation are important to know

how much of the total energy is lost before it reaches the

conversion plant. The design may have it as a “traditional

factor” of 10%, but in reality it may be higher than 15 %, or

half more than designed and expected. The energy loss

from transportation cannot be recovered but it should be

efficiently designed, built, and known. Avoid sharp 90°

elbows, avoid orifices, avoid leaks, etc.

2.1.2 Separated Brine: It can carry from 15–50% of the produced energy.

Therefore some consideration has to be given to this as an

energy carrier in liquid form, and if some of its energy can

be used. Brine can be used for electricity production,

drilling or a number of direct uses, before being reinjected.

There are available technologies to recover the brine

energy. Since this is comparably at much lower enthalpy

than steam, brine is called a low enthalpy energy source.

2.1.3 Some of the Recovered Energy can become MWe. If prioritized, some saved MWh can be directly converted

to MWe, either by using the existing power plant equipment

or by using a lower enthalpy energy conversion system.

Specifically the energy from separated brine,can become

the source of second flash steam or it can be used as heat

source for a binary system. The first option is more

economical than the ORC, since it involves no additional

equipment - the same power plant is used for the

conversion. The second option of using the hot fluid for an

ORC binary system, involves adding a new plant, with

characteristics of taking the energy from a low enthalpy

source, and therefore requires a large flow, and is of less

efficiency.

Since hot water, at the same pressure and temperature as

steam, has about 3 times less energy, or enthalpy, the

efficiency of the converting system, will be conversely 3

times less than that for the binary ORC. For example a

condensing turbine is 18% efficient, while an ORC binary

may well be operating at 6%.

2.1.4 Steam Venting and Ejectors for Vacuum These are the modern equivalents of bleeding patients.

Such practices can be avoided with more controls and better

design. They may be acceptable if used as emergency

measures and procedures, but not as continuous daily

operational ones.

Know your energy balance, efficiency, and keep walking.

3. INNOVATION IS THE KEY TO EFFICIENCY

The key to competitiveness for any economy in the world is

knowledge, and that means R&D for innovations.

4. FOUR AREAS OF MODERN GEOTHERMAL

POWER PLANT ORGANIZATION There are 4 defined areas in a modern geothermal power

plant. A person can only be in one at a time. There are

human specialists for each area. The O&M manager of a

power plant is the director of all other groups. A modern

geothermal power plant concept is useful for designing /

planning / operations organizational concept. The four

areas are:

In addition to O&M there is a Monitoring & Evaluation

A1

Reservoir + Field: gathering production + separation + venting transportation +

reinjection

A2

Energy conversion plant, turbine + generator + condenser + vacuum +

cooling system + pumps + fans

A3

Electric yard / delivery to grid. Substation +

transformer + tower + meter

A4

Control room + auxiliaries. Operators control +

maximize production

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(M&E) operation. M&E reports the results, deviations and

proposed solutions. This is a management tool for power

plant optimization.

5. LOG P VRS h – PSYCHROMETRIC CHART FOR

GEOTHERMAL USES The psychrometric charts describe the field of engineering

concerned with the determination of physical and

thermodynamic properties of vapor mixtures. It shows in a

single chart all physical conditions: P, T, density, enthalpy

and entropy.

Steam + brine mixture is precisely the case of geothermal

fluids when reaching the surface, in most cases. This is

then directed to near or far steam and brine geothermal flow

separators.

Any geothermal fluid process, and its related energy -

starting from meteoric rain thousands of years ago, to its

heating, while reaching depth, reservoir conditions, later

extraction by well casing, surface conditions, separation,

transport to electric conversion plant, and ambient release

by use of cooling tower - can all be shown in a single chart

with geothermal units – log P vs h.

It is an incredible, useful and economic tool. It is different

from other forms of modeling, like conceptual, or

mathematical, or computer aided, and other much costlier

options.

The log P h chart for surface geothermal conditions, is

presented in Figures 1 and 2. The chart is a selected section

of mixture with enthalpy h unit modification of the Mollier

log P versus h chart.

6. WASTED STEAM CONDENSATE WATER The quantity of steam condensate water that is traditionally

lost to ambient using cooling tower technology, as cooling

system, is by design about 5.6 t/MWe, every hour.

Geothermal steam water condensate may have little or no

value, when in an island, where they are surrounded by it,

for example, in Japan, Iceland, Indonesia, Philippines, etc.

Since the inception of the first generation of geothermal

power plants, cooling tower technology became acceptable

in most cases, even in drier or drought prone latitudes. In

light of the experience gained in ARGeo countries and

other locations during the past decades, it is now time to

review and reconsider this before building new plants,

Steam condensate water is very valuable:

1 MWe loses 1.4 million m3 of water.

1,000 MWe lose 1,400 million m3 of water.

As shown above, the large amount of water alone is a clear

reason to modify existing plants and change design of new

ones.

Steam condensate water can be used further instead of

evaporating it into the cooling tower plumes. It can be used

for reinjection, drilling programs, and other valuable direct

uses – like food growth. Furthermore more its value in

terms of $/m3, makes a case for its recovery and

sustainability.

An important lesson learnt in the past decades is that water

is the only “transporter” of geothermal energy that is

needed in a sustainable cycle, as reinjection, to go down

and grab more energy to bring to the surface for electric

conversion, and back down again. Many known geothermal

places go to great distances to get more reinjection flow up

to 100% of production or more.

A second option is to put a value to water condensate $/m3.

3, 5, 10? If produced from sea it will cost about $20/m3.

A third way is to see the benefit of reinjection, since it will

directly affect the reservoir pressure, for sustained

production in the long-run.

After these considerations, are seriously made, then decide

if other non-evaporative cooling technologies, such as

plumes can to be chosen. Even if the initial cost is higher,

it becomes a marginal issue, compared to the larger benefit.

(See the references for an e-link for water recovery

technology, already used at plants with hundreds of MWe).

All energy conversion systems need cooling. However,

there is a misunderstood apparent benefit of evaporating

water to ambient and saving some KWh, while the steam

water condensate could be better saved and re-used for a

long time. In some countries, like UK, and perhaps others,

the steam water plumes of cooling towers are forbidden by

law.

In Kenya, the binary plants, in Olkaria, 50MWe + 50MWe

and the thermal plants in Kipevu, 120MWe have a cooling

system that could be but is non evaporative.

Step two: Prepare a study to know the amount of water that

is being lost to ambient, from the steam condensate, by

cooling system.

Range from 5.6 – 8.5 t/h per MWe.

7. GEOTHERMAL ENERGY ECONOMIC FACTS -

IN TERMS OF COST / BENEFIT

It has been shown that a geothermal well of 5MWe

capacity, with an initial commercial cost of 6.5 M$, can last

for 30 years, if reservoir pressure is well sustained. It is

also a lot more economical than using a diesel engine

generator that will consume more than 7.5 M$ / year of fuel

to operate and produce the same electric energy of 5MWe.

Comparison of heat source cost favor geothermal 35:1.

Including O&M

Diesel engine fuel alone conversion is about 546 KWe/bbl.

At 110 $/bbl the fuel cost per 1 MWe + O&M.

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Geothermal well including O&M, the cost in 30 years for a

well head unit per MWe.

Comparison factor remains in favor of geothermal 18:1

Geothermal Energy Recovery Energy recovery has a more dramatic cost/benefit analysis -

better than the above reviewed facts of the already clear

cost/benefits of geothermal energy compared to other

sources.

The plan to do geothermal energy recovery has a cost and

benefit, in $ terms, during 30 years. Instead of wasting the

energy, doing a controlled second flash steam recovery for

1 MWe, or 8 t/h, it has a cost of about 250K$. While the

benefit, for a low 90 $/MWe, is 21.6M$.

Cost Benefit of geothermal energy recovery up to 86:1

That is a ROI of about 288% per year.

8. CONCLUSIONS

With results of sections 2-7, above, for the ARGeo

geothermal field of your choice, you have the basic inputs required to estimate and prepare the following:

+ Plan to recover energy, based on a study that will

calculate total MWh, and with MWe actual production,

calculate the actual efficiency %.

+ Recovery plan to save steam water condensate, based on

a study will determine the present steam mass losses to

ambient in total t/h.

+ Increase overall efficiency %.

+ Make money in the process.

Geothermal efficiency is profitable: The Value of the

investment cost /benefit can be overall up to 50:1.

Steam water condensate saving is better than if

released to the atmosphere. In $/m3 terms, or in pressure sustainability terms, or in reinjection cycle terms, et al.

Research & development is the solution to innovative

ways of being more competitive, efficient and adding value.

REFERENCES GEA Power cooling Systems: GEA Power Cooling, with

over 30 years experience, incorporates leading technology and lifetime customer support in its wet and dry cooling solutions, providing superior performance and years of cost effective service with minimal maintenance requirements.

Dry Cooling Solutions • Air-Cooled Condenser (ACC) • Parallel Condensing Systems

http://www.geapowercooling.com/opencms/opencms/g

pc/en/products/Air_Cooled_Condensers/

Ngure, S., KenGen: Strategic Role of Legal & Regulatory Environment, ICS Core Program on Geothermal Energy, Decision Makers' Workshop on Geothermal Energy ICS-UNIDO, June, Addis Ababa, Ethiopia, (2009).

Knowledge is the currency of the future economy. EU Research, Innovation, and Science Commissioner,

2012. Innovation comes from R&D. http://europa.eu/rapid/pressReleasesAction.do?reference=S

PEECH/12/537&format=HTML&aged=0&language=EN&guiLanguage=en

Navas, H.F. courses, conferences, papers, and presentations, at several venues, in 4 Continents:

“Modern Administration of Geothermal Plants”

Trieste, Italia. “Geothermal Separators”

-Ababa, Ethiopia. “How to save a

decade in Geothermal Developments”

for Geothermal Steam Separation”.

Evaluation for Managers of Geothermal Power Plants”

Proceedings of the 4th African Rift Geothermal Conference Nairobi, Kenya, 21-23 November 2012

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Figure 1: Log P {bara} h[KWh/t] psychrometric chart – geothermal conditions reached at surface before separation.

Figure 2: Log P {bara} h[KWh/t] psychrometric chart – geothermal conditions after separation: Brine + Steam + full curve